1. Field of the Invention
The present invention relates to a digital white balance device applicable to portable image systems such as camera phones, and more particularly to a digital automatic white balance device, which is simply implemented in a digital processing scheme by employing a grey world algorithm so that real-time processing is possible without help from a separate memory or a high performance processor, and which is also operated independently of an image sensor so that image data can be processed at high speed.
2. Description of the Related Art
White balance generally refers to the ability of a camera such as a TV camera to adjust its color balance on the basis of white color when capturing an image. Light has a specific color property called a “color temperature” that is expressed in units of K (Kelvin). Even though a scene such as a landscape a viewer views is illuminated by light of any color temperature, humans automatically adjusts their color sensitivity, thus not causing a color recognition difference. However, a TV camera or a film camera is greatly affected by the color temperature since the color temperature is directly reflected in the color.
For example, when the sky is clear, the entire screen is viewed as blue since sunlight has a high color temperature. On the other hand, at about sunrise or sunset, the entire screen is viewed as red since the color temperature at that time is low. If the camera captures an image under such a condition without performing any processing on it, it causes a problem in that the entire screen is viewed as blue and then red. To overcome such a problem, before shooting, light emitted from a light source is reflected from a white paper or the like, and a projected image of the reflected light is viewed to perform RGB (Red, Green and Blue) color balance adjustment to achieve a fine white color. Since most models of household camera-integrated video tape recorders (VTRs) have an automatic white balance function, such a color balance adjustment is automatically performed in the camera.
Recently, as camera phones are increasing in demand, there is a demand for image quality as high as a digital camera. There is also a demand for an image processing IC suitable for camera phones. The white balance is one of the image processing methods for achieving a high quality image. In the case where an image is captured with a CMOS or CCD image sensor under illumination conditions of different color temperatures, the red, green and blue components of the captured image are deviated to one side since a white color is not recognized well. This causes the image color tone to be distorted. The white balance functions to compensate for the distorted color tone. Thus, the white balance device is essential in camera phones that require high image quality.
FIG. 1 is a graph showing a general two-dimensional Cb-Cr color coordinate system.
Of a number of methods to perform the automatic white balance, the simplest and easiest to implement is based on a grey world algorithm. Under the assumption that the average of color-difference components of all pixels of an image is grey, the grey world algorithm obtains pixel gains for white balance, i.e., white balance coefficients. In the RGB color coordinate system, the grey is defined as “R=G=B”.
For example, if the pixel depth of an image is 8 bits, each of the color-difference components “Cb and Cr” has a value of 128 in “an RGB-to-YCbCr color coordinate transformation equation”, as expressed in the following equation, recommended in the ITU-R BT.601. Accordingly, the white balance can be implemented by adjusting the gain of each pixel so as to minimize a distance “d” of the average color-difference value from a white balance point (Cb=Cr=128) as shown in FIG. 1.
                              [                                                    Y                                                                    Cb                                                                    Cr                                              ]                =                                            [                                                                    16                                                                                        128                                                                                        128                                                              ]                        +                          1              256                                |                                                    65.738                                            129.057                                            25.064                                                                                      -                  37.945                                                                              -                  74.494                                                            112.439                                                                    112.439                                                              -                  94.154                                                                              -                  18.285                                                              |                      [                                                            R                                                                              G                                                                              B                                                      ]                                              [                  Equation          ⁢                                          ⁢          1                ]            
There are two main conventional methods for adjusting the pixel gain for automatic white balance. One is an analog gain adjustment method, and the other is a digital image processing method. This will now be described with reference to FIGS. 2 and 3.
FIG. 2 is a block diagram showing the configuration of a conventional analog signal processor that performs white balance in an analog fashion.
As shown in this figure, the analog image signal processor for performing the analog white balance includes an analog signal processor 22, an RGB-to-YCbCr converter 23, a Cb & Cr averaging unit 24 and an RGB gain controller 25. The analog signal processor 22 captures an image of a subject passing through a lens 21, and performs signal processing on the captured image. The RGB-to-YCbCr converter 23 converts an RGB analog signal received from the analog signal processor 22 to an YCbCr signal. The Cb & Cr averaging unit 24 obtains an average value of color-difference signals among the YCbCr signals received from the RGB-to-YCbCr converter 23. The RGB gain controller 25 obtains an RGB gain on the basis of signals from the Cb & Cr averaging unit 24, and outputs the obtained RGB gain to the analog signal processor 22.
The analog signal processor 22 includes a photocell 22A, an RGB analog gain amplifier 22B, and an A/D converter 22C. The photocell 22A captures an incident image passing through the lens 21. The RGB analog gain amplifier 22B adjusts the gain of an image signal from the photocell 22A to a gain set by the RGB gain controller 25. The A/D converter 22C outputs a signal from the RGB analog gain amplifier 22B after converting it to a digital signal.
Looking into how such a conventional image signal processor performs the white balance in an analog fashion, it can be seen that it employs a feedback system so that the amplification gain for each of the RGB channels in the sensor is controlled until the average of the color-difference components is calculated and thus the white balance is completed.
Such a conventional analog white balance method is advantageous in that a relatively accurate operation is possible because the analog gain is controlled, but has the following problems. In the case where the sensor has no port for controlling the analog gain or there is no gain control function embedded in the sensor, such a conventional method is not applicable, and the implementation thereof is also difficult if gain characteristics of the analog amplifier are not correctly known.
FIG. 3 is a block diagram showing a conventional image signal processor that performs white balance in a digital fashion.
The conventional image signal processor as shown in this figure performs digital white balance. This processor employs a different method from that of FIG. 2 to implement automatic white balance. In other words, a white balance algorithm is programmed, and a digital signal processor (DSP) or a micro controller is employed to implement the automatic white balance.
Such a conventional image signal processor for performing the white balance in a digital scheme includes an image sensor unit 32, a high-performance digital signal processor (or micro controller) 33, and a memory 34, as shown in FIG. 3. The image sensor unit 32 captures an image of a subject incident through a lens 31. The high-performance digital signal processor 33 controls the gain of an image signal from the image sensor unit 32. The memory 34 stores temporary image data and a program for real-time image processing in the digital signal processor 33.
To implement a viewfinder function of a camera, a frame rate of 30 frames/sec or more is required for an image of 300,000 pixels or more, whereas a frame rate of at least 15 frames/sec is required for an image of a million pixels or more. To realize such a high frame rate, a high-performance digital signal processor or a microcomputer capable of operating at high speed is required as shown in FIG. 3, and an additional memory may also be required as circumstances demand.
Such a conventional digital processing scheme has an advantage in that, since a digital signal processor (DSP) or a microcomputer is used, the algorithm can be implemented with a program, thereby increasing system flexibility. However, the conventional digital processing scheme has a problem in that it is not suitable for application to systems such as portable image systems including a camera phone in which smaller size and smaller power consumption is required, and the system price is also increased since it uses a high-priced general-purpose processor.